1-hydrazinophthalazine (hydralazine), its anti-hypertensive effectiveness and its side-effects are known (Ehrhard/Ruschig: Arzneimittel, 2nd Edition, Volume 2, pages 278/79, Verlag Chemie, Weinheim/Bergstrasse, 1972). Anti-hypertensive properties are also ascribed to hydralazine derivatives described in U.S. Pat. No. 3,838,125, U.S. Pat. No. 3,954,754 and in British Patent Specification No. 1,475,590.